It has until recently been generally accepted that the efficacy of discharge lamps inevitably decreases as the lamp size or wattage is reduced. As a result of this belief, discharge lamps for general lighting applications have not been developed in miniature sizes. However in the pending application of Daniel M. Cap and William H. Lake, Ser. No. 912,628, filed June 5, 1978, titled "High Pressure Metal Vapor Discharge Lamps of Improved Efficacy", which is assigned like this application, now U.S. Pat. No. 4,161,672, new miniature discharge lamps having envelope volumes of about 1 cubic centimeter or less are disclosed. These lamps have input ratings starting at about 100 watts and going down to less than 10 watts. They utilize fillings preferably comprising mercury and metal halides and have characteristics including life durations making them suitable for general lighting purposes. While the input wattage is reduced, ratios of arc watts to electrode watts similar to those in larger sizes of lamps are maintained by increasing the mercury vapor pressure at the same time as the discharge volume is decreased. It is necessary to maintain the desired electrode temperature with the reduced energy input, and this is achieved primarily by reducing the physical size of the electrodes and inleads in order to reduce the heat loss from them.
In lamps wherein the electrodes do not carry electron emission material in the conventional sense of alkaline earth metal or oxides, the criteria for the size of wire used for the overwind on the electrodes are much more critical than in lamps containing emission material. In the latter, the overwind is simply of a wire size which is adequate to retain mechanically the emission material, and which does not overheat by joule effect and cause excessive vaporization of emission material at the operating current. By way of example, in a mercury vapor lamp containing barium oxide as emission material, the electrode temperature should not exceed 1500.degree. K. and the wire size should be chosen accordingly. By contrast, in lamps without emission material or relying only upon plating out of thorium from thorium iodide in the fill for electrode activation, a temperature of 2500.degree. to 3000.degree. K. must be achieved in order to obtain adequate electron emission at a work function of 3.5 to 4.5 volts (by comparison with a work function of 1.5 to 2 where BaO is present). However at temperatures above 3300.degree. K., tungsten vaporizes at such a rate that the small envelope of the miniature lamp blackens rapidly. Accordingly the design must achieve the desired operating modes within these narrow temperature constraints. Thus a much more critical problem is presented which is made even more acute by the small size of the lamp.